Anatomy Chapter 27: Human Growth and Development

embryology

study of embryos and their development

developmental biology

a broader science that embraces changes in form and function from fertilized egg through old age

definition of embryo used by us

an individual 16 days old that consists of the three primary germ layers

three primary germ layers

ectoderm, mesoderm, endoderm

embryogenesis

events leading up to this stage

preembryonic stage

first 16 days after fertilization

when must the egg be fertilized

12 to 24 hours after ovulation if it is to survive

sperm must encounter the egg somewhere

in the distal one third of the uterine tube

vast majority of sperm

dont make it to the egg

-destroyed by vaginal acid or drain out

-fail to penetrate mucus of cervical canal

-destroyed by leukocytes in uterus

-half go up the wrong uterine tube

-of the 300 million, only 2000 to 3000 reach the vicinity of the egg

how do sperm move

lashing of tail as they crawl along the female mucosa

how does the female physiology help sperm

cervical mucus guide them through cervical canal

-uterine contractions suck semen from vagina and spread it throughout the uterus

-chemical attractant molecules released by egg attract sperm from short distance

spermatozoa reach uterine tube within

-5 to 10 minutes of ejaculation but cannot fertilize the egg for 10 hours

plasma membrane of sperm is toughened by

cholesterol

-prevents premature release of acrosomal enzymes while sperm is still in male which prevents enzymatic damage to sperm ducts

sperm must undergo

Capacitation to make it possible to fertilize an egg

-female fluids leach cholesterol from sperm plasma membrane

-dilutes inhibitory factors in semen

-sperm membrane becomes fragile and permeable to Ca2+

-diffuses into sperm causing more powerful lashing of tail

outcome of sperm capacitation

1. Swimming activity is greater = better chance of reaching oocyte!

2. Acrosome of sperm is weakened and can form a better bond with oocyte

Sperm remain viable for up to ___ days after ejaculation

6

-conception is optimal if sperm are deposited a few days before ovulation to 14 hours after

when sperm encounters an egg, it undergoes an

acrosomal reaction (exocytosis of the acrosome releasing the enzymes needed to penetrate an egg)

-enzymes of many sperm are released to clear a path for the one that will penetrate the egg

the sperm cell penetrates

granulosa cells then the zona pellucida

2 acrosomal enzymes released

hyaluronidase

acrosin

hylauronidase

digests the hyaluronic acid that binds granulosa cells together

acrosin

a protease similar to trypsin

fertilization

combines haploid sperm and egg to produce a diploid set of chromosomes

polyspermy

fertilization by more than one sperm

mechanisms to prevent polyspermy

fast block and slow block

fast block

: binding of the sperm to the egg opens Na+ channels in egg membrane

• Inflow of Na+ depolarizes membrane and inhibits the attachment of any more sperm

slow block

involves secretory granules, cortical granules, just below membrane

• Sperm penetration releases an inflow of Ca2+

• Stimulates cortical reaction in which the cortical granules release their secretion beneath the zona pellucida

• The secretion swells with water, pushes any remaining sperm away, and creates an impenetrable fertilization membrane between the egg and the zona pellucida

fertilization steps

1) sperm undergo capacitation as they migrate to oocyte

2) acrosomal reaction releases enzymes from the head of sperm

3) sperm binds to the plasma membrane of oocyte

4) sperm entry stimulates cortical reaction which destroys sperm-binding receptors

5) additional sperm cells are rejected

6) sperm nucleus swells to form a male pronucleus

7) oocyte completes meiosis II, and the ovum's nucleus swells to form a female pronucleus

8) a spindle forms between male and female pronuclei, and the chromosomes intermix

secondary oocyte begins

meiosis II before ovulation

-completes only if fertilized

-through formation of second polar body, the fertilized egg discards one chromatid from each chromosome

sperm and egg nuclei

swell and become pronuclei

each pronuclei

ruptures and the chromosomes of the two gametes mix into a single diploid set

processes in preembryonic stage

cleavage, implantation, embryogenesis

cleavage

mitotic divisions that occur in the first 3 days while the conceptus migrates down the uterine tube

-first occurs 30 hours after fertilization

during the first cleavage

zygote splits into two daughter cells (blastomeres)

by the time the conceptus arrives in the uterus

within 72 hours of ovulation

morula stage (16 cells)

still no larger than zygote

cleavage produces smaller and smaller blastomeres

morula lies

free in uterine cavity for 4 to 5 days

-divides into 100 cells or so

-zona pellucida disintegrates and releases conceptus (now blastosyst)

blastocyst

hollow sphere

trophoblast

outer layer of squamous cells in a blastocyst

-form placenta

-play role in nourishment of embryo

embryoblast

inner cell mass

-destined to become the embryo

blastocoel

internal cavity of blastula

implantation

process of attachment to uterine wall

-begins when blastocyst adheres to the endometrium

blastocyst attaches to uterine wall after

6 days

-usually on the fundus or posterior wall of uterus

Trophoblasts on attachment side separate into

two layers

superficial layer and deep layer

superficial layer

in contact with endometrium

-plasma membranes break down

-trophoblastic cells fuse into a multinucleate mass: syncytiotrophoblast

deep layer

close to embryoblast

-cytotrophoblast

cytotrophoblast

retains individual cells divided by membranes

syncytiotrophoblast

grows into uterus like little roots

-digesting endometrial cells along the way

-endometrium reacts by growing over blastocyst and covering it

-conceptus becomes buried in endometrial tissue

implantation takes about

1 week

-completed about the time the next menstrual period would have started had the woman not become pregnant

trophoblast secretes

human chorionoic gonadotropin

hCG

stimulates corpus luteum to secrete estrogen and progesterone

progesterone suppresses

menstruation

-levels rise until second month of preganancy

trophoblast develops into membrane called

chorion

-takes over role of corpus luteum making hCG unnessecary

-ovaries become inactive for remainder of pregnancy

-estrogen and progesterone levels rise from chorion

chart on slide

21

embryogenesis definition

arrangement of blastomeres into three primary germ layers

ectoderm

integument, cutaneous glands, and nervous system

mesoderm

differentiates into mesenchyme

-muscle, bones, blood

endoderm

digestive and respiratory tract

look at table

27.2

embryonic stage

begins when all three primary germ layers are present

-usually 16 days after conception

placenta

forms on uterine wall over the next six weeks

-primary source of nutrition for placenta

organogenesis

process where germ layers differentiate into organs and organ systems

-all organs at end of 8 weeks (embryo becomes fetus)

-organs are not functional

accessory organs to fetal development

placenta, umbilical cord, embryonic membranes

embryonic membranes

amnion, yolk sac, allantois, chorion

amnion

transparent sac that develops from epiblast

-completely encloses embryo

-filled with amniotic fluid

amniotic fluid

-protects embryo from trauma, infections, and temperature fluctuations

-allows freedom of movement important to muscle development

-enables embryo to develop symmetrically

-prevents body parts from adhering to each other

-stimulates lung development as fetus breathes fluid

how is amniotic fluid formed

filtration of mother's blood plasma at first

-fetus urinates into the amniotic cavity about once per hour contributing substantially to fluid volume

-fetus swallows amniotic fluid at same rate

how much amniotic fluid is there at term

700 to 1000mL

yolk sac

arises from hypoblast cells opposite amnion

-small sac suspended from ventral side of embryo

-contribute to formation of GI tract, blood cells, and future egg or sperm cells

allantois

begins as an out-pocketing of the yolk sac

-forms the foundation for the umbilical cord

-becomes part of the urinary bladder

chorion

outermost membrane enclosing all the rest of the membranes and the embryo

-has shaggy outgrowths (chorionic villi) around entire surface

-as pregnancy advances, villi of placental region grow and branch while the rest of them degernerate: smooth chorion

-villous chorion

villous chorion

at placental attachment

-forms fetal portion of placenta

conceptus is nourished in three different ways

uterine milk, trophoblastic nutrition, and placental nutrition

uterine milk

glycogen-rich secretion of the uterine tubes and endometrial glands

-absorbs this fluid as it travels down the tube and lies free in the uterine cavity before implantation

trophoblastic nutrition

conceptus consumes decidual cells of the endometrium

-only mode of nutrition for the first week after implantation

-remains dominant source through end of 8 weeks

-wanes as placental nutrition increases

progesterone from corpus luteum stimulates

decidual cells to proliferate

-they accumulate a store of glycogen, protein, lipids

-as conceptus burrows into endometrium, the syncytiotrophoblast digests them and supplies the nutrients to the embryoblast

placental nutrition

nutrients diffuse from mothers blood through the placenta into the fetal blood

placenta definition

disc-shaped organ attached into the uterine wall on one side

the other side is attached to fetus by umbilical cord

placental phase

period beginning week 9

-sole mode of nutrition from end of week 12 until birth

chorionic villi are filled with

fetal blood and surrounded by maternal blood

-bloodstreams do not mix

how thick is the placental barrier

3.5um (half the diameter of a red blood cell)

as the placenta grows, the villi grow and

branch

-their surface area increases

-membrane becomes thinner and more permeable

placental conductivity

increases: the rate at which substances diffuse through the membrane

-oxygen and nutrients pass into fetal blood

-fetal wastes pass the other way and are eliminated by the mother

the placenta is permeable to

nicotine, alcohol, and most other drugs that may be present in the maternal blood stream

final stage of prenatal development

fetal stage

-week 9 until birth

-organs mature and support life outside mother

4 weeks

vertebral column and nervous system begin to form

-limbs represented by small buds

-heart beats around 22 days

-no visible eyes, nose, ears

12 weeks

-eyes well developed (eyelids fused)

-nose develops bridge

-external ears present

-limbs well formed

-digits with fingernails

-fetus swallows amniotic fluid and produces urine

-movements but weak

-liver is prominent, produces bile

-sexes can be distinguished

20 weeks

Fine hair (Lanugo)

Cheeselike covering (vernix caseosa)

Brown fat forms (postpartum heat!)

Bent in "fetal" position- crowded!!

Mother can feel movements

"quickening"

Typical time that ultrasounds are performed to determine sex

lanugo

fine hair of baby

vernix caseosa

cheesy substance covering the skin of the fetus

28 weeks

-eyes fully open, eyelashes formed

-full head of hair

-vertex "upside-down" position

-testes begin to descend into scrotum

-typically viable if born